Posted
by
Soulskill
on Wednesday June 10, 2009 @12:30PM
from the for-music-that-really-sucks dept.

KentuckyFC writes "One of the many curious properties of Bose Einstein Condensates (BECs) is that the flow of sound through them is governed by the same equations that describe how light is bent by a gravitational field. Now, a group of Israeli physicists have exploited this idea to create an acoustic black hole in a BEC. The team created a supersonic flow of atoms within the BEC, a flow that prevents any phonon caught in it from making headway. The region where the flow changes from subsonic to supersonic is an event horizon, because any phonon unlucky enough to stray into the supersonic region can never escape. The real prize is not the acoustic black hole itself but what it makes possible: the first observation of Hawking radiation. Quantum mechanics predicts that pairs of phonons with opposite momentum ought to be constantly springing in and out of existence in a BEC. Were one of the pair to stray across the event horizon into the supersonic region, it could never escape. However, the other would be free to go on its way. This stream of phononic radiation away from an acoustic black hole would be the first observation of Hawking radiation. The team hasn't gotten that far yet, but it can't be long now before either they or their numerous competitors make this leap."

The mind-control signals travel through several million miles [wikipedia.org] of vaccuum before entering Earth's atmosphere. What's a few more millimeters? Also, tinfoil actually amplifies the signals to the point of driving the wearers mad, so nobody believes them about the aliens. Of course, it's not really aliens on Cruithne, it's actually a secret government facility tha*&(#)$@#&) ***NO CARRIER***

This is so cool. First of all I want this stuff for cavity insulation in my flat.
Then I can play music all night without next door complaining.
Having said that the BEC stuff might be either very loud or very hot, depend
on the frequency spectrum it emits, BEC loudspeakers might be possible
by varing the heat input to the supersonic regions.
However since the apparatus only hold a tiny ammount of rubidium atom, in
a magnetic trap. It might be long time before its practice.

Hmm... I can only judge it by my shot Bose amp, and no, it doesn't make a sound when you switch it off. Actually, it seems to be permanently on, because even when I flip the switch you still don't hear anything.

That's somewhere in between a metaphor for Hawking Radiation and the real thing. It's not true HR, but it would be a nice demonstration if they were to get it to work, especially if they could show some sort of analog to black hole "evaporation," which is the main implication of HR. I suppose that should naturally happen as the separation of the pairs sucks energy from the BEC and slows the fluid inside, shrinking the event-horizon-analogue.

That's somewhere in between a metaphor for Hawking Radiation and the real thing.

Not a physicist, but here's how I think the metaphor between the experiment and the real thing is supposed to work:

Speed of light: maximum speed information can travel through a vacuum ("the void")Speed of sound: maximum speed information can travel through a medium composed of atoms ("substance")

(When aircraft go supersonic, the air they run into is incapable of "preparing" to be hit, in a manner of speaking...)

We can't create stuff that goes faster than the speed of light, but we can create stuff that goes faster than the speed of sound. And just as you can't go fast enough to come back through an event horizon, information can't propagate fast enough in the experiment to go back across the subsonic/supersonic boundary. This shows us what it looks like to be in a situation like that of a black hole.

By the way, there's a similar, cheaper experiment you can do: pop a hole in a pressurized container. The gas cannot escape it (at the outlet) faster than the local speed of sound, which is obtained whenever the ratio of pressure inside to pressure outside exceeds a critical value. One gas dynamics professor said I can think of it like this: "even though a higher pressure ratio creates a greater pressure potential difference, the gas inside the tank cannot 'learn' of the greater difference because that would require information to go *into* the tank, *against* the gas that is escaping at the speed of sound"

We can't create stuff that goes faster than the speed of light, but we can create stuff that goes faster than the speed of sound.

We can't create stuff that goes faster than the speed of light in a vacuum. We create things that travel faster than the speed of light in other media all the time. The blue Cherenkov Radiation glow in fission reactors is caused by particles exceeding the speed of light in water, and creating a light shockwave analogous to the sound shockwave that e.g. supersonic aircraft produce.

Arguably, light travels faster in a Casimir cavity than in a vacuum. Really, there's no reason to suppose that "emtpy space" represents the medium through which light flows the fastest, merely that it's somewhere close.

Really, there's no reason to suppose that "emtpy space" represents the medium through which light flows the fastest

There kind of is. When light travels through a medium it is slowed down due to interactions with particles. The less it interacts with particles the faster the light travels. The photon's themselves don't ever actually change speed. How do you suppose that a substance could cause the light to travel faster than the speed of the photons themselves?

The GP's point in referring to a Casimir cavity is that space is not empty. That there is a Casimir effect proves that there is a lower density of particles in the Casimir cavity than in normal vacuum. So if you are correct in your claim;

The less it interacts with particles the faster the light travels.

then light should travel faster in a Casimir cavity then in vacuum. Though, I would bet that it still does not allow you to send a signal faster than with light in vacuum.

No, the casimir cavity has less energy than a vacuum. Negative, as it were.

That is not to say that photons would move faster in a casimir cavity. I have no idea whether they would, but it would be easy to calculate - does the permeability of the vacuum differ from that of a casimir cavity?

We can't create stuff that goes faster than the speed of light in a vacuum.

That's what "the speed of light" means. People almost never mean "the speed of light in water" (or some other non-vacuum medium) when they just say "the speed of light" unless they've previously established the context earlier.

We create things that travel faster than the speed of light in other media all the time. The blue Cherenkov Radiation glow in fission reactors is caused by particles exceeding the speed of light in water, and creating a light shockwave analogous to the sound shockwave that e.g. supersonic aircraft produce.

When you say "we", I'm presuming you're referring to all those of us who have fission reactors giving off Cherenkov Radiation glow in our back sheds...

One gas dynamics professor said I can think of it like this: "even though a higher pressure ratio creates a greater pressure potential difference, the gas inside the tank cannot 'learn' of the greater difference because that would require information to go *into* the tank, *against* the gas that is escaping at the speed of sound"

I really don't like that explanation... it makes it seem like the pressure differential is "known" to the gas inside the cylinder via some sort of acoustically-transmitted information. My initial reaction was "HUH" and my secondary reaction was "ok, I don't buy that."

I really don't like that explanation... it makes it seem like the pressure differential is "known" to the gas inside the cylinder via some sort of acoustically-transmitted information. My initial reaction was "HUH" and my secondary reaction was "ok, I don't buy that."

That's almost what he's saying. He's not saying there is acoustically-transmitted information, but that there is physically transmitted information.

Imagine you are inside the container. If you measure how much air is leaving the container, you have access to all the information required to know the pressure of the outside. At a certain point, the pressure of the outside has no further effect on the flow, so inside of the container, no further information about the outside is possible. You only know that the

So while the external pressure is changing, no information about that fact is making its way into the container!

True enough, but why not? That's really the whole question.

So the physical/mechanical transfer of information cannot exceed the speed of sound in this system,

Why not? Why is the speed of sound the limiting factor?

if the air is leaving at the speed of sound, trying to go backwards at the speed of sound results in a net speed of zero.

I return to the thin-plate scenario: a thin plate should still obey your rules. Air is escaping at the speed of sound. If the flow of information is asymptotically limited by the speed of sound, the velocity shouldn't increase beyond that, no matter how thin the plate is. It does.

The acoustic properties of solids are generally pretty good. Again, I don't see the thickness of the plate really playing much of a role in determining whether or not information can "get inside" the tank.

We can't create stuff that goes faster than the speed of light, but we can create stuff that goes faster than the speed of sound. And just as you can't go fast enough to come back through an event horizon, information can't propagate fast enough in the experiment to go back across the subsonic/supersonic boundary. This shows us what it looks like to be in a situation like that of a black hole.

That's exactly right. What they have done is create an acoustic event horizon. It doesn't hold all the same properties as a real black hole, but as Korn says, there is a chance that you can see Hawking Radiation and possibly BH evaporation from this experiment. There has already been a paper [arxiv.org] suggesting that you can see Hawking radiation by looking at the density correlation functions of the BEC.

That's somewhere in between a metaphor for Hawking Radiation and the real thing.

This isn't really a metaphor exactly. If the equations governing two systems are the same, then we expect the behavior to be the same, and we can describe them in the same terms. Phonons themselves are a good example of this: a phonon is hardly the sort of thing that you would intuitively think of as a particle, but because the equations governing phonons are the same as those governing quantum mechanical particles, physicists describe phonons as particles. Subatomic particles themselves bear very little

That's somewhere in between a metaphor for Hawking Radiation and the real thing.

This isn't really a metaphor exactly. If the equations governing two systems are the same, then we expect the behavior to be the same, and we can describe them in the same terms. Phonons themselves are a good example of this: a phonon is hardly the sort of thing that you would intuitively think of as a particle, but because the equations governing phonons are the same as those governing quantum mechanical particles, physicists describe phonons as particles. Subatomic particles themselves bear very little resemblance to the 'billiard ball' particles that most people imagine. I think that it would be better to say that Hawking radiation is just an effect predicted for systems obeying certain equations, and in that sense, both the acoustic and traditional black holes exhibit completely real Hawking Radiation.

It is true that getting 'acoustic Hawking radiation' wouldn't constitute absolute proof that Black Holes do the same thing - our model may be wrong. What it will do do is provide proof that, assuming our model is correct, Hawking radiation is real, and there isn't some unanticipated effect which invalidates the theory.

I take your point, and you may easily have more expertise than I do (non-specialist grad quantum mechanics classes and a couple undergrad astro classes along with some casual enthusiasm for the subject). My understanding of Hawking radiation is that the split virtual pair explanation isn't physically accurate, but that tunneling of particles through the event horizon is the more physically valid explanation.

1) I'm not aware of an analogous effect that will work for phonons. Tunneling itself is on the wron

1) I'm not aware of an analogous effect that will work for phonons. Tunneling itself is on the wrong lengthscale.

2) Since the virtual particle pair splitting explanation also satisfies the radiation equations, maybe this difference doesn't have much significance. Were someone to convince me of this, I'd fully agree with you.

Well, I'll admit that my knowledge of Phonons is actually fairly limited, so I'm trusting the article in it's claim that the equations governing the two systems are the same. If not then the article is being very misleading, and I'd have to change my stance! My own personal opinion about quantum mechanics though is that the equations themselves represent the fundamental thing, and the particular interpretation you place on them is just a convenience.

The real prize is not the acoustic black hole itself but what it might makes possible: the first observation of something analogous to Hawking radiation. The Theory of Quantum mechanics predicts that pairs of phonons with opposite momentum ought to be constantly springing in and out of existence in a BEC. Theoretically, were one of the pair to stray across the event horizon into the supersonic region, it could never escape. However, the other would be free to go on its way. This stream

"Phonos" are basically "crystal oscillations". Enter the concept of "reciprocal space": it's basically the Fourier transform of the real 3D space, and is very commonly used in solid state physics.

Now as you probably know, a clean frequency (i.e. a sinus wave) in the time domain results to a single peak in the Fourier-Transform (i.e. in the frequency domain). And similar for phonons: a clean crystal oscillation (i.e. a single-frequency sound wave propagating through a medium) in 3D space results in the equiv

No, phonons are indeed quantum mechanical. (A phonon is essentially the joint wavefunction describing many different nuclei in a solid.) The main difference that I see between this setup and a black hole is in the "vacuum" from which particles are created. In a solid, phonons are typically created by a myriad of scattering events. Two electrons could scatter off each other, an electron could scatter off a nucleus, a photon of visible light could make dozens of phonons, etc. Near a black hole, though, virtual pairs need to be created spontaneously from the vacuum. So, the upshot is that while the general mechanism is the same in both cases, I would guess that phonons in a BEC are created far more frequently than virtual pairs near a black hole.

I would hope we can use it to defend our civilization from both outside and inside attack. Maybe put a few of them in orbit that would suck in the drudgery of the reality shows and entertainment-news talking heads so that no outside civilization will feel the need to demolish the planet to build a hyperspace bypass. Plus, we won't have to listen/watch this crap anymore. A world without Fox/CNN/MSNBC... wow. I can only hope.

Yes, when will humanity invent this mysterious technology that allows us to avoid reality TV and cable news? This device, known to SciFi writers as the "off switch" may forever be the stuff of fiction.

Black holes in a Bathtub by E. Berti (2005):
www.iop.org/EJ/article/1742-6596/8/1/013/jpconf5_8_013.pdf

The argument basically goes that when you unplug your bathtub, there's a certain point at which waves generated past the "event horizon" near the hole never escape the hole. It's an interesting read, but I was under the impression that this is basically the same thing, albeit not an effect that arises from quantum field theory.

My father (who happens to be almost deaf) has a unique approach to this that has seen good results through the years... He keeps a couple of CDs of music with him that are frankly just awful. When someone is being obnoxious with their music, he switches away from whatever he was normally listening to, takes his hearing aids out, selects the song(s) he think will cause the recipient the most auditory distress, and then cranks his music until his seat rattles... leaving it turned up until they're out of heari

"Nigel Tufnel: The numbers all go to eleven. Look, right across the board, eleven, eleven, eleven and...
Marty DiBergi: Oh, I see. And most amps go up to ten?
Nigel Tufnel: Exactly.
Marty DiBergi: Does that mean it's louder? Is it any louder?
Nigel Tufnel: Well, it's one louder, isn't it? It's not ten. You see, most blokes, you know, will be playing at ten. You're on ten here, all the way up, all the way up, all the way up, you're on ten on your guitar. Where can you go from there? Where?
Marty DiBergi: I don't know.
Nigel Tufnel: Nowhere. Exactly. What we do is, if we need that extra push over the cliff, you know what we do?
Marty DiBergi: Put it up to eleven.
Nigel Tufnel: Eleven. Exactly. One louder.
Marty DiBergi: Why don't you just make ten louder and make ten be the top number and make that a little louder?
Nigel Tufnel: [pause] These go to eleven."
"Nigel Tufnel: It's like, how much more black could this be? and the answer is none. None more black. "

Now, this is sounding like Yahoo Serious - Young Einstien (real Stupid Funny movie) Were talking splittling beer atoms to power the worlds first electric guitar, and lets not forget the oversized speaker from Back to the Future. Let me tell you there is NO WAY that an accoustic black hole can withstand a 8' speaker, fed from an amp that goes to eleven, and powered by beer atom fission! We're talking, fucking, muscle.

How do you infer from the effects of phonons, that the same happens to photons? If they had the same effects, this would mean that luminiferous aether would exist. Which as far as we know, is not true, and replaced by the theory of relativity. Or would it be the effect of a quantized space-time? And would those quantums then be some kind of particles?

i think the key is trusting the equations.ftfs: One of the many curious properties of Bose Einstein Condensates (BECs) is that the flow of sound through them is governed by the same equations that describe how light is bent by a gravitational field.

but w/r/t the aether, i'm kind of intrigued recently by this guy [youtube.com] who posits that the aether theory is in fact correct and that the mistake in the michaelson/morley era was assuming that matter was not itself a [standing] wave propogating though the aether.

we all know photon pairs are connected. an observation on changes the state of the other. (quantum entanglement) but what happens if one of a pair of photons enters a black hole and the other remains outside?

does quantum entanglement still exist fo these photons? does the photon still exist inside the black hole or does it disintegrate or change state? if so: what would happen to the other photon outside?

Nah, Bose doesn't suck so much as you can get similar quality gear for a better price. It's swanky audio gear for people who don't know anything about audio gear.

Depends on which end of the spectrum you're at.

At the entry-level end, if you're just talking about speakers in a system, it's just a matter of choice. My four matched Bose 201's and VC-10 centre channel cost neither more nor less than other speakers that were available at the time. To my ear, they sound just fine, and I'd know people whose Bose

The reality is, most people don't have super-discriminating ears, and are therefore not really capable of telling much difference between any speakers.

You don't need super-discriminating ears to know that Bose systems sound like shit. It's perfectly obvious. Bose doesn't even do well against inexpensive systems, let alone ones in the Bose price-range.

You know, sound is entirely subjective. Bose figures that to much of the listening public, a shift to the mid range is more pleasing to the ear. ("No highs, no lows, must be Bose" I believe is how it gets phrased.)

They've been in business for over 40 years. Clearly, a measurable percentage of people who aren't you, disagree with you.

They've been in business for over 40 years. Clearly, a measurable percentage of people who aren't you, disagree with you.

Bose used to make some decent products, but not anymore. Current buyers are simply victims of duplicitous marketing (lies, in other words).

In the end, do you really give a shit that I think my speakers sound fine and you feel they could only sound like crap?

I don't care about your tastes, personally. What I do care about is a company that is a bunch of lying parasites. I believe in honest business, and think that liars and scammers should be out of business, not making ridiculous profits based on ignorance.

So, your choice in speakers matters only insofar as you are enabling the scum.

I don't have any experience of Bose speakers, but I have a pair of their noise cancelling headphones and used to own a pair of their earbuds - both of which were gifts, I hasten to add. Much as I love music, I am not a hi-fi buff and wouldn't spend £250 on headphones. Still, there have been innumerable occasions when I have heard details in familiar songs I had never noticed before in hundreds of plays. The headphones (QuietComfort 3s) are a little bass-heavy, if anything, but the buds sounded to me a

But Soundgarden already did (or at least traveled through one). Unfortunately, the singer's voice was permanently damaged by the experience, and so when he sings "Black Hole Sound" we hear "Black Hole Sun" instead.

Or something.

Not that Lemmy doesn't have more talent in his superfluous third nipple than Soundgarden has, but he probably just hasn't bothered with an acoustic black hole yet.